The present invention relates to an electric spark machining apparatus and, particularly, to such apparatus equipped with a cooling means for cooling machining liquid to control a temperature thereof to an optimum value for a specific machining.
FIG. 1 shows, schematically, a typical example of a conventional electric spark machining apparatus having a ccoling means. In FIG. 1, the electric spark machine 1 is associated with a machining liquid supply device 2 for supplying the machining liquid to the machine 1. Used liquid in the machine 1 is returned through a drain pipe 101 to the liquid supply device 2, pumped by a pump 3 to a filter 4 and filtered thereby. A cleaned liquid 21 is returned through a supply pipe 6 to a machining portion of the electric spark machine 1 by a pump 5. In such circulation of the machining liquid, it is heated by electric discharge energy and/or driving energy of the pumps 3 and 5, which may affect a machining accuracy of products adversely. Therefore, the machining liquid must be regulated in temperature, suitably.
A cooling means is used to realize the regulation of machining liquid temperature. The cooling means includes a cooling device 7 and a pump 22 for supplying the liquid 21 to the cooling device 7. The cooling device 7 includes a heat-exchanger tank 71 and a compressor 72 for compressing a coolant of the cooling device.
The cooling device 7 further includes a first temperature sensor 8 for sensing a temperature of the machine 1 or an ambient temperature, a second temperature sensor 9 for sensing the temperature of the liquid in the liquid supply device 2, a comparator 10 for comparing an output of the first sensor 8 with that of the second sensor 9 to provide a temperature difference signal and a control device 11 responsive to the difference signal from the comparator 10 to on-off control the compressor 72 to thereby regulate the liquid temperature to a preset value.
With such cooling means, it is possible to control the temperature of liquid 21 in the liquid supply device 2. However, liquid 21 is heated by the pump 5 and, particularly, the temperature of a portion of the liquid 21 which passes through a supply portion of liquid directly connected to the machine 1 tends to vary with variation of shaft power of the pump 5 or variation of flow-rate of the liquid, causing a stabilization of the temperature of the liquid to be fed to the machining portion of the machine 1 to be impossible.
Further, since an amount of liquid 21 in the liquid supply device 2 is considerable, there is a considerable time lag in a response of the temperature control to a variation of the output signal of the sensor 8, which makes the liquid temperature control insufficient.